The present invention relates to the use of a nonionic alkylhydroxyalkyl cellulose to improve gloss and printability of a surface, for instance paper or cardboard, which is coated with a coating composition containing calcium carbonate and a latex. Advantageously, the non-ionic alkylhydroxyalkyl cellulose can be combined with a carboxymethyl cellulose.
When coating, such as smearing or surface sizing of cellulose-based surface-shaped products, for instance paper or cardboard, with a composition the primary aim is to change the properties of the paper product, such as improved strength, improved appearance, improved properties of printability, improved impermeability or improved properties of adhesion. Thus, it is common to surface size paper or cardboard with an aqueous binder-containing composition in order to improve, inter alia, wet strength or to coat paper or cardboard with a composition that contains pigment pastes in order to improve the printing properties and the appearance of the surface. The coating compositions may, apart from binders and pigment, also contain secondary binders (co-binders), protective colloids, thickening agents and dispersing agents. It is common to add water-soluble or water-swellable polymers as thickening agent, protective colloid or secondary binders, on the basis of polyvinyl alcohol, modified celluloses, starch, casein, alginate or high molecular carboxyl-group-containing polymerizate.
EP 15 517 discloses the use of cellulose ethers in coating mixtures, where carboxymethyl cellulose is added as a secondary binder. It is already known from EP 307 795 to use, inter alia, methyl cellulose as a protective colloid. EP 425 997 discloses that hydrophobically modified alkyl cellulose, alkylhydroxyalkyl cellulose or hydroxyalkyl cellulose can be used as a thickening agent in aqueous paper coating compositions. The hydrophobic groups are preferably C12-16 alkyl groups or alkyl aryl groups. When used in coating compositions these cellulose ethers, when compared with carboxymethyl cellulose ethers, give an improved viscosity at high shear rates.
From EP 496 269 it is also known to prepare a polysaccharide-containing suspension for paper coating, the suspension containing a low-molecular polysaccharide which is dissolved in aqueous phases and may be a carboxymethyl cellulose or a hydroxyethyl cellulose. Moreover, the coating composition contains one or more dispersed, i.e. not dissolved, cellulose polymers, such as hydroxypropyl cellulose, methyl cellulose, methylhydroxypropyl cellulose and hydrophobically modified hydroxyethyl cellulose.
WO 97/46757 discloses the use of a water-soluble non-ionic alkylhydroxyalkyl cellulose which contains alkyl groups having 1-3 carbon atoms and hydroxyalkyl groups having 2-3 carbon atoms but which is free from hydrophobically modified hydrocarbon groups having at least 4 carbon atoms, the cellulose ether having a turbidity point in the range 35-80xc2x0 C., as a thickening agent in aqueous compositions for coating cellulose-based surface-shaped products. By using the cellulose ethers as a thickening agent, process engineering advantages are obtained when applying the coating compositions to surface-shaped cellulose products.
The present invention generally relates to the use of a non-ionic alkylhydroxyalkyl cellulose, which has alkyl groups having 1-3 carbon atoms and hydroxyalkyl groups having 2-3 carbon atoms, is free from hydrophobically modifying hydrocarbon groups having at least 4 carbon atoms and has a turbidity point of 50-95xc2x0 C., or a combination of the alkylhydroxyalkyl cellulose with a carboxymethyl cellulose, where the amount of the carboxymethyl cellulose is 1200 percent by weight at the most of the amount of the alkylhydroxyalkyl cellulose in an aqueous coating composition containing a calcium carbonate and a latex in order to improve the gloss and the printability of the surface which is obtained when paper or cardboard is coated with the coating composition.
According to the present invention, it has now been found that the use of a non-ionic alkylhydroxyalkyl cellulose, which has alkyl groups having 1-3 carbon atoms and hydroxyalkyl groups having 2-3 carbon atoms, is free from hydrophobically modifying hydrocarbon groups having at least 4 carbon atoms and has a turbidity point of 50-95xc2x0 C., or a combination of the alkylhydroxyalkyl cellulose with a carboxymethyl cellulose, where the amount of the carboxymethyl cellulose is 1200 percent by weight at the most of the amount of the alkylhydroxyalkyl cellulose, in an aqueous coating composition containing a calcium carbonate, and a latex considerably improves the gloss and the printability of the surface that is obtained when paper or cardboard are coated with the coating composition.
Extensive studies have shown that the use of the alkylhydroxyalkyl cellulose in a latex coating composition which is pigmented with calcium carbonate gives an improved gloss and changes in the surface of the coating layer, such as higher hydrophilicity, higher surface energies and a higher relative polarity. These changes of the surface lead to improved printing properties. For example, the gloss, set-off and smoothness of the surface are improved. It has also been found that the carboxymethyl cellulose in combination with the alkylhydroxyalkyl cellulose gives a surprisingly good result, although the use of the carboxymethyl cellulose only gives a considerably lower hydrophilicity, surface energies, relative polarity, gloss and printability than the alkylhydroxyalkyl cellulose. Since the carboxymethyl cellulose in relation to the alkylhydroxyalkyl cellulose is easier to prepare and, moreover, is produced on a large scale, using a combination of carboxymethyl cellulose and alkylhydroxymethyl cellulose means a commercially attractive embodiment of the invention. It has also turned out that the coating composition which contains both an alkylhydroxyalkyl cellulose and a carboxymethyl cellulose according to the invention exhibit excellent rheology properties compared to a coating composition which contains carboxymethyl cellulose only. For instance, the viscosity of a coating composition containing the combination is less dependent on the shear rate than may be expected. If a combination of alkylhydroxyalkyl cellulose and carboxymethyl cellulose is used, the amount of carboxymethyl cellulose usually constitutes 20-1200 percent by weight, preferably 50-600 percent by weight, of the amount of the alkylhydroxyalkyl cellulose.
The viscosity of the alkylhydroxyalkyl cellulose may vary considerably and should in a conventional manner be adapted to the composition of the coating mixture. Usually it is in the range 5-100,000 mPaxc2x7s, preferably in the range 10-10,000 mPaxc2x7s, measured in a Brookfield viscosimeter of the type LV in a 2% solution at 20xc2x0 C. Examples of suitable alkylhydroxyalkyl celluloses are ethylhydroxyethyl cellulose, methylethylhydroxyethyl cellulose, methylethylhydroxy-ethylhydroxypropyl cellulose, methylhydroxyethyl cellulose and methylhydroxypropyl cellulose. Usually the hydroxyethyl groups constitute at least 30% of the total number of hydroxyalkyl groups and the number of ethyl substituents usually constitute at least 10% of the total number of alkyl substituents. Examples of such cellulose ethers are ethylhydroxyethyl cellulose with DSethyl=0.8-1.3 and MShydroxyethyl=1.9-2.9 and methylethylhydroxyethyl cellulose with DSmethyl=1.0-2.5; DSethyl=0.1-0.6 and MShydroxyethyl=0.1-0.8. The amount of alkylhydroxyalkyl cellulose is usually from 0.05 to 3, preferably from 0.2 to 1.5 percent by weight of the composition.
The carboxymethyl cellulose usually has a degree of substitution of carboxymethyl of 0.6xe2x88x921.5. It can exist in acid form, but preferably the salt form with a preferably monovalent cation, such as sodium, is used. The carboxymethyl cellulose usually has a somewhat lower viscosity than the alkylhydroxyalkyl cellulose and it is normally in the range 2-15,000 mPaxc2x7s, preferably in the range 5-5000 mPaxc2x7s, the viscosity being measured in the same manner as for the alkylhydroxyalkyl cellulose. Apart from carboxymethyl groups, the carboxymethyl cellulose can also contain other substituents, such as hydroxyethyl groups and methyl groups.
As already mentioned, the coating composition according to the invention is pigmented with calcium carbonate but can, advantageously, also contain other pigments. Examples thereof are kaolin, talc, titanium oxide, satin white, hydrated aluminium, sodium silicoaluminate, gypsum and plastic pigment and a large number of special pigments, such as barium sulphate and zinc oxide. The total amount of pigment in the coating composition is usually 5-65 percent by weight and may constitute 4 to 20 times the amount of weight of latex. Of the total amount of pigment, the amount of calcium carbonate normally constitutes 5-100 percent by weight.
The composition according to the invention also contains a latex binder, which can be any conventional latex binder, conveniently in an amount of 2xe2x88x9270, preferably 5xe2x88x9230, percent by weight based on the weight of the coating mixture. The latex may consist of copolymers which are made of monomers from the group ethylenically unsaturated carboxylic acids, esters of acrylic acid and methacrylic acid, acrylonitrile, methacrylonitrile, acrylamide, methacrylamide, C3-C5 ethylenically unsaturated monocarboxylic acids, ethylenically unsaturated dicarboxylic acids and their halfesters, vinyl chloride, ethylenically unsaturated hydrocarbons, vinyl ester, vinyl sulphonic acid and esters of unsaturated carboxylic acids which are derived from polyvalent alcohols. Examples of suitable copolymers are polymers of vinyl acetate and acrylate, copolymers of vinyl acetate, ethylene and vinyl chloride, copolymers of styrene and acrylates, copolymers of styrene and butadiene and copolymers of styrene, butadiene and acrylonitrile and mixtures thereof. Said copolymers can also contain further monomers, such as esters of unsaturated carboxylic acids and unsaturated carboxylic acids can be copolymerised. It is also possible to use homopolymers, for example of acrylic acid and vinyl esters. As supplementary binders to the latex binders, sometimes called secondary binders, also polyvinyl alcohol, modified cellulose starch, casein, alginate, cellulose esters and high molecular carboxylic containing polymers can be added.
A typical aqueous coating composition for use according to the invention has the following composition based on its dry content:
0.07-5, preferably 0.3-2 percent by weight of the alkylhydroxyalkyl cellulose or the alkylhydroxyalkyl cellulose and the carboxymethyl cellulose at the weight ratios defined above.
4-15, preferably 7-13 percent by weight of latex,
60-94, preferably 70-90 percent by weight of pigment, of which 5-100 percent by weight is calcium carbonate,
0-10, preferably 0-7 percent by weight of secondary binder,
0-3, preferably 0.1-2 percent by weight of dispersant,
0-5, preferably 0-2 percent by weight of protective colloid,
0-4, preferably 0-2 percent by weight of other additives,
such as optical whitener, bactericides, pH-regulating agents, antifoam agent and lubricant, the dry content being 20-72 percent by weight. The Brookfield viscosity of the coating composition is usually 100-2500 mPaxc2x7s at 100 revolutions/minute and at 20xc2x0 C. The viscosity of coating compositions with the dry content of 55-72 percent by weight is 600-2500 mPaxc2x7s. An aqueous coating composition for use according to the invention can be made by the pigment part being dispersed in water, possibly by means of a dispersant. To the obtained pigment wash, one may then add secondary binders, alkylhydroxyalkyl cellulose, carboxymethyl cellulose and other additives included in the composition and, finally, latex, preferably in the form of a dispersion.
A cellulose-based product coated with the above-mentioned composition can be prepared by
a) applying the composition on the cellulose-based product at a temperature below the turbidity point of the alkylhydroxyalkyl cellulose,
b) drying the surface-shaped cellulose product by heating, the cellulose product being coated with the composition, and
c) calendering the cellulose product, if desired.
The application of the composition takes place in a manner known per se, for instance, by air knife coating, roller coating or blade coating. Owing to season and geographical location, the temperature of the composition is usually in the range 5-55, preferably 20-40xc2x0 C.
The invention is further illustrated by the following examples.